1. Field of the Invention
A proven method for reducing pollutants, in particular for reducing nitrogen oxides in the exhaust of lean-operated internal combustion engines, is the reduction of nitrogen oxides by means of a suitable chemical reducing agent. The known methods share the feature that fluid, (e.g. liquid or gaseous), pollutant-reducing media are sprayed into an exhaust system of an internal combustion engine where they react with the pollutants in the exhaust and chemically convert them. For this reason, a metering system for metering such fluid, pollutant-reducing media is proposed below, which system can be used for reducing pollutants in motor vehicle exhaust.
2. Prior Art Description of the Prior Art
In the so-called ammonia SCR method, particularly in the commercial vehicle sector, ammonia as a reducing agent is required in the exhaust upstream of a corresponding reducing agent catalyst. In current methods, the actual reducing agent ammonia is often released through thermolysis and subsequent catalyzing hydrolysis by air-injecting an aqueous urea/air aerosol into the engine exhaust through the use of compressed air.
With regard to a possible use in passenger vehicles, however, this method with compressed air injection is considered to be ill-adapted to market requirements due to its high system complexity (cost, space required, compressed air supply).
Another method for avoiding dependence on compressed air provides for the injection of a urea solution (AdBlue) into the engine exhaust. This is accomplished, for example, by means of pumps for pressurized delivery of the AdBlue.
This often involves the use of valves of the kind known from gasoline injection, which have various disadvantages. With these valves, difficulties arise for example due to the necessity for cooling the (usually electrical) valves, which are mounted directly onto the hot exhaust system. In addition, the winter operability of such systems is considered to be problematic. At temperatures below −11° C., the conventional urea/water solution freezes and expands. Consequently, achieving a winter operability of such valves and of other involved system components (pumps, pressure regulation, etc.) requires a considerable degree of engineering effort and expense, which also increase the system complexity (e.g. a construction that is able to withstand ice pressure, reverse-suction pumps, etc.).
DE 196 46 643 C1 has disclosed a system for injecting a nitrogen oxide reducing agent into an exhaust gas flow. In this case, the reducing agent is controllably injected into the exhaust gas flow in the form of fine jets delivered through a plurality of fine nozzle openings by means of local, cyclical generation of excess pressure. In this case, the nozzles are embodied in the form of piezoelectricaily controlled nozzles similar to the nozzles used in piezoelectric inkjet printing heads or nozzles with cyclically operated heating resistor elements that are similar to known bubble jet-type inkjet printing heads. The system described in DE 196 46 643 C1, however, has a complex apparatus, is prone to malfunction—particularly with regard to corrosive fluids, and is expensive.
DE 103 24 482 A1 has disclosed controlling a metering valve, which is supplied from the return of a reducing agent circuit, by setting a certain pulse-width ratio. The reducing agent circuit in this case can be regulated and switched on and off by means of a pressure regulating valve. The valve is mounted onto the exhaust pipe and it is necessary to install and use mechanically rugged, short circuit-protected electrical lines and plug connections between a control unit and the valve. The electrically switchable valve is also subjected to the heat of the exhaust pipe and can be cooled by means of a fluid circuit in the form of the reducing agent circuit.